Methods for making optical gratings have developed from two primary starting points. First, mechanical ruling or dividing is utilized where a machine indexes the cutting of finely separated lines on a substrate. Second, interference patterns are generated optically and are reproduced in a photographic substance such that the patterning, for instance of a photoresist is accomplished by imaging an interference pattern onto the photoresist. Development of the photoresist leaves a highly defined pattern, which is then utilized as a mask for vapor deposition, the pattern of which forms a transmission grating. It will be appreciated that the latter technique has been successfully developed with the aid of monochromatic light sources such as lasers. However, the grating lines are in general curved, corresponding to the concentric circles of the interference pattern. With respect to mechanical ruling or dividing, this is a time consuming process requiring extreme control and has, for the most part, been replaced by the optical generation technique described above.
There are many varieties of gratings, but they can be divided into two basic types: reflection and transmission. There are also many variations in groove or line profile to enhance the quality of the grating, which techniques include echelon, blazed, and laminar approaches. It will be appreciated that all of these gratings fall into a category relating to the modification of a surface. Either the surface of a substrate is itself modified or the grating is produced by the modification of a coating on the surface of a substrate.
Gratings of the transmissive type have been used as optical couplers for the coupling of light into and out of optical waveguides which form so-called optical integrated circuits. One such technique is described in the Journal of Applied Physics Letters, June 15, 1970, Vol. 16, No. 12 entitled "Grating Coupler for Efficient Excitation of Optical Guided Waves in Thin Film", pps. 523-525. In this article a method of coupling a laser beam into a thin film optical waveguide utilizes an optical grating that is made utilizing photoresist and is fabricated directly on the top surface of the optical waveguide. The grating is provided, in one embodiment, by the aforementioned photolithographic technique in which an interference pattern is imaged on a photoresist which lies directly on the glass film serving as the optical waveguide. As noted in this article, the potential for producing miniaturized optical integrated circuits on a single wafer which is resistant to vibrations and thermal effects has raised the need for a simple, efficient means of coupling light into and out of thin film waveguides. Previous coupling techniques for such an application include edge illumination and evanescent field coupling with a prism.
While photolithographic techniques utilizing interference patterns have produced acceptable gratings in the past, it will be appreciated that the interference rings produced through interfering one laser beam with another are generally curved in nature. Moreover, it is oftentimes inconvenient to pattern a grating directly on the optical integrated circuit described above.